L-methionine, an essential amino acid in a living body, has been used as feed, infusion, a medicinal raw material such as a synthetic raw material for pharmaceutical drugs, and food additives. Methionine is an important amino acid involved in transmethylation in vivo and has a role of providing sulfur.
For the chemical synthesis of methionine, a method to hydrolyze 5-(β-methylmercaptoethyl)-hydantoin to produce methionine in the form of a mixture of L-type and D-type is mainly used. However, the chemical synthesis results in a production of methionine in the form of a mixture of L-type and D-type. Meanwhile, L-methionine may be selectively produced using a biological method, in which L-methionine is produced by direct fermentation using a microorganism or by a two-step process (International Patent Publication No. WO 2008/013432). Specifically, the two-step process consists of a process for producing L-methionine precursor by fermentation and a process for converting the L-methionine precursor to L-methionine by an enzyme. The two-step process can selectively produce only L-methionine and further produce simultaneously organic acids, more specifically succinic acid or acetic acid as byproducts by the same reaction.
The L-methionine precursor may include O-acetylhomoserine and O-succinylhomoserine, and the enzymes used in the conversion process may include O-succinylhomoserine sulfhydrylase and O-acetylhomoserine sulfhydrylase. To maximize L-methionine production by the two-step process, it is necessary to secure the maximal amount of fermented O-acyl homoserine, which is a precursor of L-methionine. Simultaneously, the enzymes used for enzyme conversion to produce L-methionine must have high conversion activity, exhibit overexpression in microorganisms, and maintain a high reaction rate at high concentrations of O-acyl homoserine. Additionally, the enzymes must have low activity-inhibition at a time point when the final product (i.e., L-methionine and organic acid) is accumulated at high concentration and have thermal stability not to lose their activity during reactions. In this regard, KR Patent No. 10-1250651 discloses O-acetylhomoserine sulfhydrylase derived from Rhodobacter sphaeroides, which has higher thermal stability than O-acetylhomoserine sulfhydrylases derived from microorganisms other than Rhodobacter sphaeroides, and is thus newly available for use in the two-step process. However, there is still a need to develop an enzyme, which has high conversion activity for use in the two-step process, etc. and low activity-inhibition at a time point when the final products are accumulated.